Simulation of cryogenic silicon etching under SF₆/O₂/Ar plasma discharge

An etching simulator is developed to study the two-dimensional (2D) silicon etch profile evolution under SF₆/O₂ inductively coupled plasma discharge. The simulator is composed of three modules: plasma kinetic module, sheath module, and etching module. With this approach, the authors can predict the 2D etch profile evolution versus reactor parameters. Simulation results from the sheath model show that the shape of the bimodal ion energy distribution function for each incident angle depends on the ion mass. It is all the larger that the ion mass is low. As shown in the experiment, the simulation results reveal that the atomic oxygen plays an important role in the passivation process along the side-wall. Indeed, the simulation results show the decrease of the undercut when the %O₂ increases. This improves the etching anisotropy. However, the decrease in the etch rate is observed for a high %O₂. Moreover, for a moderate direct current (DC) bias (some 10 V), a low variation of the silicon etch profile versus DC bias is observed. The moderate ion energy only allows removing of the passivation layer on the surface bottom. The etching process is mainly controlled by the chemical etching under fluorine flux.